1. Field of the Invention
The invention relates to an image printing apparatus comprising a number of image forming units for forming toner images of different colors in alignment on an image receiving material in accordance with electronic image information signals fed to each image forming unit. Each image forming unit includes an image forming element rotatable about an axis of rotation and provided with a dielectric surface layer with electrode tracks therebeneath. Each image unit further includes developing means comprising a linear developing magnet situated near the outer surface and parallel to the axis of rotation of the image forming element, toner feed means for feeding electrically conductive and magnetically attractable toner powder to the image forming element, and electrode track activating means. The electrode activating means applies a first or a second printing voltage between an electrode track and the magnet system in accordance with the image information signals to be fed to the image forming units. Depending on the voltage, powder on the passage of the electrode track along the developing magnet toner either remains on the image forming element or does not remain thereon in the event of a first or second printing voltage respectively on the electrode track.
2. Description of Background Art
An image printing apparatus is described in European patent EP 0 310 209 in which, by the use of known toner feed means, toner powder is brought into contact with the image forming element from a toner reservoir over an entire image dimension. In a first embodiment, the image forming element is constructed as a rotatable cylindrical element. By maintaining a minimum feed voltage difference between the electrode tracks and the toner feed means the dielectric surface layer of the image forming element is charged via the electrically conductive toner powder. The toner powder in contact with the surface layer in these conditions is then retained by the resulting charge build-up in the dielectric surface layer. This part of the image forming element thus provided with an even layer of toner powder then passes a linear magnetic field formed by the linear developing magnet, the magnetic field extending axially and being sharply defined in the direction of rotation. The magnetic force exerted as a result is sufficient to draw the magnetically attractable toner powder from the image forming element. In one embodiment, the developing means are constructed as a linear magnet disposed to be stationary and extending over the entire axial direction of the image forming element, with a rotating sleeve extending therearound, the direction of rotation thereof being opposite to that of the image forming element. European patent EP 0 304 983 describes developing means of this kind. The toner powder drawn from the image forming element by the magnetic field will be transported back to the toner reservoir by the rotating sleeve.
If an extra voltage difference is now applied between the magnet system and the image forming element, the extra electric force exerted thereon can cause the toner powder to be retained on the image forming element, despite the magnetic force exerted thereon by the magnet system. This printing voltage can be applied for a short printing period to any electrode track. As a result, the toner coverage on the image forming element is limited by the electrode track in the axial direction and by the printing period in the direction of rotation. Thus a printing voltage applied simultaneously to all the electrode tracks for a minimum printing period results, for example, in a toner coverage in the form of a line parallel to the axis of rotation of the image forming element.
A toner image thus developed by an image forming unit can be transferred by suitable toner transfer means to the image receiving material, with or without the use of an intermedium. This transfer can be effected by pressure or by electrostatic transfer. The European patent EP 0 373 704, for example, describes an embodiment with pressure transfer and an intermediate element.
Particularly in the case of color printing it is important to obtain an accurate positioning of the different color separation images, such positioning being constant over the entire image dimension. In the case of a mixed pattern consisting of different adjacent toner coverages in different colors, an undesirable color shift occurs when these areas cover one another. Alternatively, where two surfaces each of a different color adjoin, an undesirable boundary effect occurs or else the color of the image receiving material between the surfaces becomes visible.
In the case of pressure transfer with an intermediate element as described in European patent EP 0 373 704, in principle no toner powder is transferred from an image forming element over an existing layer of toner already applied by another image forming unit on the intermediate element. In this case, the balance of the mechanical and electrical forces is such that toner is transferred to the intermediate element if the latter is not yet covered with toner but remains on the image forming element if that is the case. In the event of incorrect mutual positioning of toner separation images, there is in that case the risk that less toner of a specific color will be transferred as a result of overlap.
The accurate positioning of color separation images is a known problem for which various solutions have been developed.
U.S. Pat. No. 4,931,815, for example, describes a color printing apparatus which successively prints a number of color separation images directly in alignment on a paper sheet. For this purpose, the paper sheet is conveyed along a straight transport path passing a number of cylindrical drums disposed along the path and provided with a photoconductive surface layer. A color separation image is developed on each photoconductive drum by means of a laser scanner, drum charging means and toner feed means. The color separation images developed thereon are then electrostatically transferred to the paper sheet.
The color printing apparatus described in the above patent is provided with a number of position detectors in the form of CCD's which are located after the photoconductive drums in the downstream direction and in which each CCD covers a part of the transport path. Each photoconductive drum then prints on the transport path a line parallel to the axis of the drum. The position detectors then detect any deviation in the direction of the transport path between the lines placed by the various photoconductor drums. The magnitude of this deviation is stored by means of a counter for each photoconductive drum. The time at which the image information to be fed to each laser scanner is read out is then corrected for this deviation. Consequently, the leading edges of each color separation image coincide, at least insofar as they are parallel and identical. Any skewing or bending of an image line reproduced by the photoconductive drums is not corrected as a result. All the image signals to be fed to the scanner means of a photoconductive drum in fact undergo the same shift.
U.S. Pat. No. 5,384,592 also describes a color printing apparatus with means for correcting imperfect alignment of the color separation images. By exposure means such as a laser scanner or an LED array, a charge image is formed for each color on a cylindrical photoconductor. Toner feed means are used to form toner covering corresponding to the charge image, either directly on a paper sheet passing along the photoconductors by means of a conveyor belt, or indirectly by transfer to a collecting belt taken past the photoconductors. Position sensors in the form of linear CCD arrays are also disposed in the downstream direction for scanning the edges of the conveyor belt or collecting belt. By disposing markers on both edge zones by means of the various photoconductors, these position sensors can detect whether there is a relative shift in the direction of transport, skewing or a shift perpendicular to the direction of transport between markers of different photoconductors. In the case of a shift in the direction of transport, a correction is made by correcting the starting signal for an image. In the case of skewing, correction is obtained, in the case of apparatus with laser scanning means, by turning a deflection mirror, and in the case of an arrangement with LED array, by turning the array with automatic movement means. In the case of a shift perpendicular to the direction of the transport, a start signal for an image line is corrected.
However, none of the color printing means described in the above patents can correct any deformation of an image line or skewing without additional displacement means. Although the latter patent points out that the markers must not be disposed too far apart at the two edges in view of the presence of any bending perpendicular to the direction of transport, it gives no complete image line correction of this bending. It is however precisely in the case of a developing magnet of the kind referred to in the preamble that considerable requirements must be satisfied in respect of the straightness of the magnetic field generated thereby, since this is largely responsible for the straightness of an image line produced as a result. It must also be remembered that a minimum dimension of a toner covering reproduced in the direction of transit can at most be just a few tens of micrometers in the case of an image resolution of 1600 dpi. There is also a disturbance of the imaging by the toner displacement between the developing magnet and the image forming element.